Metal substrates including metal oxide nanoporous thin films and methods of making the same

ABSTRACT

The present disclosure is directed to a metal-containing apparatus including a substrate member constructed of a metal that is highly resistant to pitting corrosion and wear in aggressive media. An exemplary metal-containing apparatus is a plate heat exchanger. The metal includes an oxidation layer on the surface thereof and a thin metal oxide nanoporous film on top of the oxidation layer. The nanoporous film is highly compliant and is comprised of oxygen and aluminum, titanium, silicon, zirconium and combinations thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 14/669,820, filed Mar. 26, 2015, which is acontinuation application of U.S. patent application Ser. No. 12/503,150,filed Jul. 15, 2009 (now U.S. Pat. No. 8,993,131), which claims priorityto U.S. Provisional Patent Application Ser. No. 61/081,271, which wasfiled on Jul. 16, 2008, the entirety of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present disclosure is directed to highly corrosion andwear-resistant metal substrates having an oxidized surface layer havingchemically bonded thereto a nanoporous thin film. More particularly, thepresent disclosure is directed to highly corrosion and wear-resistantmetal substrates, such as stainless steel, carbon steel, or aluminum,that have an oxidized surface and a metal oxide nanoporous thin filmlayer chemically bonded to the oxidized surface that is prepared from ananoparticulate sol of titanium dioxide, silicon dioxide, zirconiumdioxide, alumina or a combination thereof. The present disclosure isalso directed at various methods for preparing the highly corrosion andwear-resistant metal substrates including the metal oxide nanoporousthin film.

In many applications that utilize metal substrates that are in contactwith aggressive media such as corrosive liquids, the metal substratescan quickly become corroded and worn. One specific example of a metalsubstrate that can quickly become corroded and worn during normal usageis a metal plate heat exchanger. Metal plate heat exchangers arecommonly constructed of stainless steel, although in applications wheresalt and chlorides are present, more expensive materials such astitanium or stainless steel alloys such as 254 SMO® may be utilized.More aggressive media such as sulfuric acid and the like may requirespecial alloys. Generally, a stainless steel heat exchanger will have alife expectancy of less than about five years in many applications, andmay be less than one year in particularly corrosive applications.

The amount of corrosion and wear on a metal substrate, such as a metalplate heat exchanger, shell and tube heat exchangers, or cooling towers,for example, is directly dependent upon three variables: (1) the metalcomprising the metal substrate; (2) the surface roughness of the metalsubstrate prior to the application of any coating; and (3) the mediathat the metal substrate is exposed to. These variables need to beaddressed and controlled in order to achieve the greatest lifeexpectancy for the metal substrate during use.

As compared to other types of steel, stainless steel generally exhibitsa lower corrosion rate in an aqueous environment due to the formation ofa thin passivating oxide film that covers and protects the metalsurface. Pitting corrosion occurs when passivity fails or is otherwiseabsent at localized points on the metal surface. Pitting corrosion is aparticularly aggressive form of corrosion that is focused on a smallarea of the metal substrate. Pitting corrosion forms pits, which areholes formed on the metal surface. Pits tend to propagate very rapidlydue to anodic dissolution of the metal. As such, pitting corrosion maybe referred to as “localized.” A common and important type of pittingcorrosion occurs on passivated iron-based alloys in contact withhalide-containing solutions. Chloride is a common and aggressive halideanion and causes pitting corrosion in many metals and alloys.

Generation of corrosion pits on stainless steel immersed in aggressivemedia, such as a chloride solution, generally occurs in three distinctstages: nucleation, metastable growth, and stable growth. Many pits thatnucleate do not propagate indefinitely. Instead, many pits re-passivateafter a very short period of metastable growth. Metastable pitsgenerally do not cause significant damage to the metal surface. Thefinal diameter of metastable pits may be a few micrometers.

Pit growth is generally sustained by the development of a highlyaggressive analyte, which involves a strong oxidizing solution having anoxidizing reduction potential of +600 to +1200 mV inside of the pit. Theanalyte also comprises an enhanced concentration of anions that migrateinto the pit, which maintains analytic charge neutrality. Pit growth isself-sustaining due at least in part to development of the aggressiveanalyte.

Most pits generally tend to continue growing once they have becomeestablished. Therefore, the susceptibility of a metal to pittingcorrosion is linked to the formation of stable pits. The resistance ofstainless steel to pitting generally relates to the critical potentialmeasurable by various electrochemical methods. The potentiodynamicmethod involves an applied potential scanned to noble values, wherebythe respective current is measured.

The potentiodynamic method provides a measurement of the pittingpotential (E_(p)) related to pit nucleation. The potentiodynamic curvesdepend upon experimental variables, such as potential scanning rate.E_(p) may be determined by extrapolating to the passive current densityfor the rising curve observed in the early stages of pitting. E_(p)values may also be determined from the same curve by relating to apredetermined current density and to a current density ten times higher.

Another accelerated way of testing for pitting corrosion resistance withrespect to a surrounding medium is to expose the coated stainless steelmaterial to an aggressive corrosion medium of interest. Periodic visualexamination of the material permits qualitative ranking with respect topitting corrosion susceptibility. Although such testing is more timeconsuming than the electrochemical methods, it may provide improvedevaluation of longevity in a medium of specific interest.

Based on the foregoing, there is a need in the art for metal substratesthat are constructed out of lower cost materials that can provideincreased resistance against pitting and corrosion and thus a longerservice life.

SUMMARY OF THE INVENTION

The present disclosure provides metal substrates, such as metal plateheat exchangers and the like, that have significant resistance topitting and corrosion in aggressive media and thus provide for extendeduse without failure. The metal substrates have an oxidized layer on thesurface thereof and a very thin metal oxide nanoporous film chemicallybonded to the oxidized layer. The nanoporous film is derived from asuspension containing nanoparticles (Sol) whereby the nanoparticles arecomprised of oxygen and an element selected from the group consisting ofaluminum, titanium, silicon, zirconium and combinations thereof. Thenanoporous film has a thickness of less than about 1 micrometer and hasa porosity of from about 26% to about 80%. In some embodiments of thepresent disclosure described herein, the metal substrate includes afirst nanoporous film chemically bonded to the oxidized layer of thesubstrate and a second nanoporous film on top of the first nanoporousfilm. This first and second nanoporous film may be of the same ordifferent composition.

The present disclosure also provides various methods for preparing ametal substrate including an oxidized layer on the surface thereof and ananoporous film chemically bonded to the oxidized layer.

As such, one embodiment of the present disclosure is directed to ametal-containing apparatus comprising a substrate member constructed ofa metal, an oxidized layer on a surface of the substrate member, and afirst nanoporous film comprising oxygen and an element selected from thegroup consisting of aluminum, titanium, silicon, zirconium andcombinations thereof, the nanoporous film being chemically bonded to theoxidized layer, the nanoporous film having a thickness of less thanabout 1 micrometer and having a porosity of from about 26% to about 80%.

In another embodiment the present disclosure is directed to a method ofmaking a metal-containing apparatus, the method comprising: (1)providing a substrate material constructed of a metal; (2) introducingan oxidized layer on a surface of the substrate material, the oxidizedlayer having a thickness of less than about 200 nanometers; (3)providing a sol comprising nanoparticulate γ-AlOOH, TiO₂, SiO₂, ZrO₂, ora mixture thereof, the nanoparticulates having a primary particle sizeof from about 1.5 nanometers to about 50 nanometers; (4) contacting thesol with the oxidized layer to provide at least one layer of the solonto the oxidized layer; (5) heating the at least one layer at atemperature of from about 60° C. to about 100° C. for a time period offrom about 15 minutes to about 2 hours to produce at least one drylayer; and (6) thermally sintering the at least one dry layer to producea nanoporous film comprising oxygen and aluminum, titanium, silicon,zirconium or a combination thereof, the nanoporous film having athickness of less than about 1 micrometer and a porosity of from about26% to about 80%.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the pitting potential of various StainlessSteel 316 Coupons (B2 Finish) immersed in an Instant Ocean Solution.

FIG. 2 is a graph showing the pitting potential of various StainlessSteel 304 Coupons (B2 Finish) immersed in an Instant Ocean Solution.

FIG. 3 is a graph showing the pitting potential of various Carbon Steel(B2 Finish) immersed in an Instant Ocean Solution.

FIG. 4 is a graph showing the pitting potential of various StainlessSteel 201 (Brushed) Coupons immersed in a deaerated 1M NaCl solution.

FIG. 5 is a graph showing the pitting potential of various StainlessSteel 436 (Brushed) Coupons immersed in a deaerated 1M NaCl solution.

FIG. 6 is a graph showing the pitting potential of black rebar samplesimmersed in 0.23M NaCl solution, saturated in Ca(OH)₂.

FIG. 7 is a graph showing the pitting potential of aluminum couponsimmersed in an Instant Ocean Solution.

FIG. 8 is a scanning electron microscopy picture (SEM) of 316 stainlesssteel dip-coated with three layers of a sol comprising nanoparticulateTiO₂ (magnification of 5.00 K×), whereby each wet sol layer was dried at100° C. for 30 minutes and whereby the three layers were sintered at300° C. for one hour.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure is directed to a metal-containing apparatus, suchas a plate heat exchanger and the like, that are highly corrosion andwear resistant in aggressive media. The metal-containing apparatusincludes a substrate member constructed of a metal that has an oxidationlayer on a surface thereof and a thin, metal oxide nanoporous filmchemically bonded to the oxidation layer. The nanoporous film is formedfrom a nanoparticulate sol and includes oxygen and an element selectedfrom the group consisting of aluminum, titanium, silicon, zirconium andcombinations thereof. The nanoporous film has a thickness of less thanabout 1 micrometer and has a porosity of from about 26% to about 80%.Surprisingly, it has been discovered that a thin, porous film asdescribed herein can be utilized on the metal substrate to significantlyimprove corrosion and pitting performance and increase overall wear ofthe apparatus. Before the unexpected discovery described herein, it wasthought that these types of films needed to be thicker, non-porous filmssuch that there would not be any discontinuous areas in the film and thefilm would not allow aggressive media to pass through and contact themetal. Surprisingly, the nanoporous films described herein provide anumber of benefits while being thin and porous, and thus highlyresistant to cracking and/or fracturing.

As noted above, the nanoporous films described herein are applied to ametal-containing apparatus including a substrate member constructed of ametal. The metal-containing apparatus can be any metal containingapparatus that is subjected to aggressive media during use and may besubject to corrosion, pitting, and wear due to the aggressive media. Thesubstrate member constructed of a metal can have any thickness suitablefor its intended use. In one embodiment, a metal substrate useful in aplate heat exchanger has a thickness of from about 0.1 millimeters toabout 100 millimeters, or even 0.1 millimeters to about 10 millimeters,or even 0.1 millimeters to about 1 millimeter. Specific, non-limitingexamples of metal-containing apparatuses include plate heat exchangers,shell and tube heat exchangers, cooling towers, and the like. The metalsubstrate utilized in the metal-containing apparatus can be any metalthat is subject to corrosion in aggressive media including, for example,stainless steel (e.g., 100 series, 200 series, 300 series, 400 series,500 series, 600 series stainless steel, a 2205 stainless steel, a 304stainless steel, and a 316 stainless steel), carbon steel, reinforcingrebars, and aluminum. In some embodiments, it is preferred that thesurface roughness of the metal substrate utilized in themetal-containing apparatus be minimized in order to improve adhesion ofthe oxidized layer and metal oxide nanoporous film.

The metal substrate described above includes an oxidized layer on asurface thereof in accordance with the present disclosure. The oxidizedlayer present on a surface of the metal generally has a thickness ofless than about 200 nanometers, and generally less than about 100nanometers.

The oxidation layer is introduced onto a surface of the metal substrateused in the metal-containing apparatus to improve the bonding of thenanoporous film described herein to the metal substrate. It has beenobserved that in the absence of the deposited oxidation layer, the“bare” metal surface does not “wet” well with the nanoparticulate solutilized to deposit the nanoporous film and the resulting nanoporousfilm quality is non-uniform and poor.

The metal substrate additionally includes one or more metal oxidenanoporous films on top of the oxidation layer. As described more fullybelow, the nanoporous film is preferably compliant with respect tobending, twisting, and stretching of the substrate and uniform in natureand is introduced directly on top of the oxidation layer and results inthe nanoporous film being chemically bonded to the oxidation layer. Oncethe first nanoporous film is introduced directly on top of the oxidationlayer, one or more additional nanoporous films may be introduced on topof the first nanoporous film such that the metal substrate may includeone nanoporous film on top of the oxidation layer, two nanoporous filmson top of the oxidation layer, three nanoporous films on top of theoxidation layer, etc. If more than one nanoporous film is present on themetal substrate, the nanoporous films may be of identical construction,or may be of different constructions. In addition, each film in itselfcould contain a single type of metal oxide or multiple metal oxides.

The nanoporous film comprises oxygen and an element selected from thegroup consisting of aluminum, titanium, silicon, zirconium andcombinations thereof. By way of non-limiting example, the nanoporousfilm may comprise (1) aluminum and oxygen; (2) titanium and oxygen; (3)silicon and oxygen; (4) zirconium and oxygen; (5) silicon, titanium andoxygen; (6) aluminum, zirconium, and oxygen; or (7) aluminum, silicon,and oxygen and so forth. The nanoporous film generally has a thicknessof less than about 1 micrometer, and in one embodiment may be from about0.01 micrometers to about 1 micrometer. In another specific embodiment,the nanoporous film has a thickness of from about 200 nanometers toabout 300 nanometers. In another specific embodiment, the nanoporousfilm has a thickness of from about 0.01 micrometers to about 100nanometers. Nanoporous films greater than 1 micrometer in thickness tendto begin to take on thermo-mechanical properties of expansion andcontraction, which are generally undesirable. As such, these thickerfilms tend to crack and fracture during use. This cracking andfracturing can lead to pitting corrosion.

The nanoporous film generally has a porosity of from about 26% to about80% and, in some embodiments, has a porosity of about 26%. The porosityof the nanoporous films depend upon the particle size of thenanoparticulates making up the film, the packing of the particles in thefilm, the chemical identity of the metal oxide particle, and thesintering temperature.

In accordance with the present disclosure, methods of preparing themetal-containing apparatus are also disclosed. In one embodiment, themetal-containing apparatus is prepared by introducing an oxidized layeronto a surface of a substrate member constructed of a metal that will beincorporated into the metal-containing apparatus. The oxidation layer onthe metal substrate can be applied using a “dry oxidation” process or“wet oxidation” process. Although not required in every embodiment ofthe present disclosure, it is generally desirable that the surface ofthe substrate member constructed of a metal be cleaned/degreased priorto the growth of the oxidation layer. One suitable method forcleaning/degreasing the metal surface is by using soap and water toremove any grease/organic material from the surface onto which theoxidation will be introduced.

The “dry oxidation” process can be done by heating the metal substratein air at a temperature of from about 200° C. to about 400° C. for atime period sufficient to grow the oxidation layer to a desiredthickness. In one specific embodiment, the metal substrate may be heatedin air to a temperature of 300° C. at a rate of about 10° C. per minuteand maintained at the 300° C. temperature for about two hours. The metalsubstrate is then cooled to room temperature.

A “wet oxidation” process may also be used to provide the desiredoxidation layer on the substrate member constructed of a metal. A basicsolution such as sodium hydroxide or a solution of hydrogen peroxide maybe used to provide the desired oxidation layer. The metal substrate isgenerally held in the solution for a period of from about one hour toabout four hours. In one specific embodiment, the metal substrate isimmersed in a 6% hydrogen peroxide solution having a pH of about 13 (0.1M NaOH) for about three hours. The metal substrate is then washed withMQ water to provide the desired oxidation layer.

Once the desired oxidation layer has been introduced onto a surface ofthe substrate member constructed of a metal, a sol comprisingnanoparticulate γAlOOH, TiO₂, SiO₂, ZrO₂, or a mixture thereof iscontacted with the oxidized substrate member constructed of a metal. Thenanoparticles included in the sol generally have a particle size of fromabout 1.5 nanometers to about 50 nanometers, or even 1.5 nanometers toabout 20 nanometers, or even 1.5 nanometers to about 10 nanometers. Inone specific embodiment, the nanoporous films described herein areprepared from a sol comprising nanoparticulate TiO₂ having a primaryparticle size of from about 3 nanometers to about 8 nanometers. Inanother specific embodiment, the nanoporous films described herein areprepared from a sol comprising nanoparticulate SiO₂ having a primaryparticle size of from about 1.5 nanometers to about 8 nanometers. Inanother specific embodiment, the nanoporous films described herein areprepared from a sol comprising nanoparticulate ZrO₂ having a primaryparticle size of from about 3 nanometers to about 8 nanometers. Inanother specific embodiment, the nanoporous films described herein areprepared from a sol comprising nanoparticulate SiO₂ having a primaryparticle size of from about 1.5 nanometers to about 8 nanometers andnanoparticulate TiO₂ having a primary particle size of from about 3nanometers to about 8 nanometers.

In accordance with the present disclosure, the sols utilized tointroduce the nanoporous films onto the oxidized layer may be preparedby any suitable process known in the art. In one specific embodiment, asol comprising SiO₂ nanoparticulates may be prepared using tetraethylorthosilicate which is hydrolyzed in an ammonia based solution (such as,for example, a 0.5M solution) with stirring for about one hour. Theresulting sol may then be aged for about 24 hours at which time largerparticles or precipitates can be removed by filtration. A hydroxidesolution may then be added with stirring to produce a sol having aconcentration of SiO₂ of 30 g/L.

In another specific embodiment, a sol comprising TiO₂ nanoparticulatesmay be prepared by hydrolysis of titanium isopropoxide in a nitric acidsolution. A suitable solution may be 1.43 mL of concentrated nitric acidin 200 mL of water and 33 mL of Ti(OPR)₄. A white precipitate formsunder stirring and the resulting aqueous phase becomes transparent dueto peptization (the breaking or electrostatic segregation of largerparticles into smaller ones) under strong stirring. The resultingsuspension becomes a transparent sol, which may be placed in dialysistubing and dialyzed against pure MQ water to adjust the pH to about 3.5.The resulting sol has a solids concentration of about 25 g/L.

In another specific embodiment, a sol comprising ZrO₂ nanoparticulatesmay be prepared by the hydrolysis of zirconium propoxide in nitric acid.A suitable mixture is 3 mL of concentrated nitric acid in 150 mL ofwater and 11 mL of Zr(OPr)₄. After one day of strong stirring, a whitesuspension forms upon mixing of the propoxide, and the aqueous phasebecomes transparent due to peptization. The transparent sol may then bedialyzed in MQ water until a pH of about 3.2 is reached. The resultingsolids content is about 12 g/L.

In another specific embodiment, a sol comprising SiO₂ nanoparticulatesand TiO₂ nanoparticulates is prepared wherein the silica and titania solmixture is in a weight percentage of 70% SiO₂ and 30% TiO₂.

The contacting of the sol with the oxidized layer on the substratemember constructed of a metal to produce a wet layer of sol on theoxidized layer may be accomplished by any manner suitable in the art.For example, the sol may be contacted with the oxidized layer utilizinga process selected from the group consisting of dipping, draining,spraying, electro-deposition, spinning, and combinations thereof. Thethickness of the resulting nanoporous film on top of the oxidized layermay generally be controlled by the amount of time the oxidized layer isexposed to the contacting process and the concentration of particles inthe suspension. For example, if a dipping or dip coating process isutilized as the contacting procedure, the thickness of the resultingnanoporous film can be controlled by the rate at which the substratemember constructed of a metal is withdrawn from the dip chambercontaining a given concentration of particles in the sol. Similarly, ifa spraying process is utilized as the contacting procedure, thethickness of the resulting nanoporous film can be controlled by theamount of suspension delivered, the concentration of particles in thesuspension, and the duration of the spraying of the sol onto theoxidized surface of the substrate member constructed of a metal.

It will be recognized by one skilled in the art based on the disclosureherein that the substrate member constructed of a metal may be contactedwith the sol one time, two times, three times, four times, or moredepending on the desired end result. For example, if a dipping processis being utilized, the substrate member constructed of a metal andhaving an oxidized surface layer may be dipped into a sol one or moretimes and may have a drying process as described herein utilized betweendips such that multiple nanoporous films or layers are introduced ontothe substrate.

After the sol has been contacted with the oxidized layer on thesubstrate member constructed of a metal to form a wet sol layer on theoxidized surface, the resulting substrate member is then heated at atemperature of from about 60° C. to about 100° C. for a time period offrom about 15 minutes to about 2 hours to produce a dry metal oxidenanoporous film on top of the oxidized layer. In some embodiments, itmay not be necessary to dry at an elevated temperature and roomtemperature may be utilized for drying (for a period of a one minute ora few minutes to a few hours). Once a dry nanoporous film has beenformed on top of the oxidized layer, the substrate member constructed ofa metal may be subjected to a process for depositing another wet layerof sol on the surface of the dry nanoporous film which is then subjectedto the drying process described above. This will result in a secondnanoporous film on the surface of the substrate member constructed of ametal. The substrate member constructed of a metal may ultimately haveone, two, three, four or even more nanoporous film layers on top of theoxidized layer. These nanoporous film layers may be constructed of thesame metal oxides, or may be constructed of different metal oxides.

After the one or more dried nanoporous film(s) has been introduced ontothe oxidized layer of the substrate member constructed of a metal, thesubstrate member is then thermally sintered to produce a nanoporousfilm(s) comprising oxygen and aluminum, titanium, silicon, zirconium ora combination thereof. The sintering may be carried out at a temperatureof from about 100° C. to about 500° C., generally from about 200° C. toabout 400° C., or even 250° C. to about 350° C. for a time period offrom about 30 minutes to about 3 hours. In one specific embodiment, theupper limit of the firing temperature is just under the melting point ofthe substrate. Referring now to FIG. 8, there is shown a scanningelectron microscopy picture of a 316 stainless steel coupon dip-coated,dried and sintered in accordance with the present disclosure.

During the sintering process, there may be some elemental/atomicdiffusion between the metal oxide layer and the components of theunderlying metal. For example, iron, nickel, chromium and/or cobalt maydiffuse from the underlying metal, such as stainless steel, into themetal oxide layer, which may change the sintering properties of thenanoporous films. In particular, etching may occur at highertemperatures, whereby the iron, nickel, chromium, and/or cobalt areinserted into the lattice structure of the nanoparticulate metal oxideparticles.

With increased sintering temperatures, the nanoporous films decrease insurface area and porosity whereas the pore size increases due to themigration of material from the pore surface to the interparticle spaceto make a neck between particles. Some of the small pores are beingfilled during this process, and thus the pore size distribution movestowards larger pores. As a result, the density of the nanoporous filmsincreases with sintering. Ellipsometry testing may be used to determinethe density of the nanoporous films.

In one exemplary process of the present disclosure, a plate heatexchanger is fabricated by the following steps: (1) providing asubstrate member constructed from stainless steel; (2) oxidizing a layerof the stainless on the surface of the substrate member, the oxidizedlayer having a thickness less than about 200 nanometers; (3) providing asol comprising nanoparticulate γ-AlOOH, TiO₂, SiO₂, ZrO₂ or a mixturethereof, the nanoparticles having a primary particle size of from about1.5 nanometers to about 50 nanometers; (4) dip coating one or more wetlayers of the sol onto the oxidized layer; (5) heating the wet layer(s)at a temperature of from about 60° C. to about 100° C. for a time periodof from about 15 minutes to about 2 hours to make a dry layer; and (6)thermally sintering the dry layer to make a nanoporous film comprisingoxygen and Al, Ti, Si, Zr or a combination thereof, the film beingchemically bonded to the oxidized layer on the surface of the substratemember, and the film having a thickness of less than about 1 micrometerand having a porosity of from about 26% to about 80%.

EXAMPLES Example 1

Electrochemical testing and evaluation for pitting potential ofstainless steel 316 coupons (B2 finish) immersed in sea water (InstantOcean Solution) was conducted. The stainless steel 316 coupons (B2finish) had various surface coatings thereon as set forth below. Eachcoupon tested was a square plate of either 1 inch by two inches or oneinch by four inches.

The stainless steel 316 (B2 finish) coupons evaluated were as follows:(1) untreated; (2) surface treated with a dry oxidation procedure plus acoating of TiO₂ (70%) and SiO₂ (30%) (3) surface treated with a dryoxidation procedure plus a coating of TiO₂; (4) surface treated with adry oxidation procedure plus a coating of ZrO₂ (“A”); and (5) surfacetreated with a dry oxidation procedure plus a coating of ZrO₂ (“B”).

For the coupons treated with a dry oxidation step, the followingprocedure was used. The stainless steel coupon was heated to 200° C. ata rate of 10° C. per minute in air. The coupon was held at thistemperature for 1 hour and subsequently cooled to room temperature. Thisresulted in an oxidation layer having a thickness of about 100nanometers.

The preparation of the TiO₂ sol was done by hydrolysis of titaniumisopropoxide in a nitric acid solution including 1.43 mL of concentratednitric acid in 200 mL of water and 33 mL of Ti(OPR)₄. A whiteprecipitate was obtained upon mixing the Ti(OPr)₄ with the aqueous phasebecoming transparent due to peptization under strong stirring of thesuspension. The suspension became a transparent sol, and the sol wasplaced in a Spectra/Por dialysis tubing and dialyzed against pure water(MQ) to slowly adjust the pH to 3.5. The solids concentration was about25 g/L.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

The preparation of the SiO₂ sol was done by using tetraethylorthosilicate which was hydrolyzed in an ammonia based solution (0.5M)by stirring one hour at room temperature. The sol was then aged for 24hours, and the larger particles or precipitates were removed byfiltration. A hydroxide solution was then added with stirring. Theconcentration SiO₂ was 30 g/L.

The preparation of the TiO₂/SiO₂ was done by mixing the sols in a weightratio of 70% TiO₂ and 30% SiO₂.

Dip coating was performed on the coupons (with the exception of theuntreated coupon) to introduce the nanoporous film onto the oxidizedlayer. Each coupon was dip coated into the desired sol and withdrawn ata speed of about 3 millimeters per second. Once removed from the dipcoating chamber, each coupon was dried at 100° C. for about 30 minutesand then cooled to room temperature in air. After the drying of thenanoporous coating was complete, each coated coupon was subjected to asintering step that included heating the coupon at 10° C. per minute to300° C. and then dwelled at 300° C. for 1 hour and then cooled to roomtemperature in air. This process resulted in a nanoporous film having athickness of between about 200 nanometers and about 300 nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for each of the five coupons described above. The tests weredone in aerated solutions at a temperature of 23° C. and a pH of 8.1.

The elements of the electrochemical cell included the following: asurrogate sea water solution (“Instant Ocean”) saturated with oxygen;temperature of 23° C.; V=300 mL; a Calomel Reference electrode; and a PtCounter Electrode=Pt. The working electrode was the coupon beingevaluated. The tested area of the working electrode was 18 cm². About 3centimeters of the working electrode was immersed into the sea watersolution. 300 mL of sea water solution was added to the cell, and theelectrodes inserted into the sea water solution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to thesea water solution, the OCV became quasi-constant for the coupons. Thepotential was then increased at a constant rate of 150 mV/s until thecurrent density reached I=3 mA/cm² or until the potential reached 0.9mV.

Referring now to FIG. 1, there is shown CV curves for each of thecoupons evaluated. FIG. 1 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The untreated coupon incurred a sharp increase in currentdensity of around 0.4 V. In contrast, the coupons having a nanoporouscoating as described herein incurred a sharp increase in current densityno less than about 0.6 V and as much as 1.1 V. The curve increaseassociated with the coupons having a nanoporous coating is not as sharpas the curve for the uncoated coupon.

For the coupons having the nanoporous coating, the gradual increase inpotential is theoretically associated with nucleation of pits andformation of metastable pits. The sharp/spike increase in potential istheoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits. As such, theincrease in current density was associated with pitting and not with thetranspassive potential of the uncoated substrate metal. Transpassivepotential is associated with the point in time when general dissolutionof the passivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the various metal oxides or mixturesof metal oxides provided significant protection from pitting as comparedto untreated coupons.

Example 2

Electrochemical testing and evaluation for pitting potential ofstainless steel 304 coupons (B2 finish) immersed in sea water (InstantOcean Solution) was conducted. The stainless steel 304 coupons (B2finish) had various surface coatings thereon as set forth below. Eachcoupon tested was a square plate of either 1 inch by two inches or oneinch by four inches.

The stainless steel 304 (B2 finish) coupons evaluated were as follows:(1) untreated; (2) surface treated with a dry oxidation procedure plus acoating of ZrO₂; surface treated with a dry oxidation procedure plus acoating of TiO₂; (4) surface treated with a dry oxidation procedure plusa coating of TiO₂ (70%) and SiO₂ (30%).

For the coupons treated with a dry oxidation step, the followingprocedure was used. The stainless steel coupon was heated to 200° C. ata rate of 10° C. per minute in air. The coupon was held at thistemperature for 1 hour and subsequently cooled to room temperature. Thisresulted in an oxidation layer having a thickness of about 100nanometers.

The preparation of the TiO₂ sol was done by hydrolysis of titaniumisopropoxide in a nitric acid solution including 1.43 mL of concentratednitric acid in 200 mL of water and 33 mL of Ti(OPR)₄. A whiteprecipitate was obtained upon mixing the Ti(OPr)₄ with the aqueous phasebecoming transparent due to peptization under strong stirring of thesuspension. The suspension became a transparent sol, and the sol wasplaced in a Spectra/Por dialysis tubing and dialyzed against pure water(MQ) to slowly adjust the pH to 3.5. The solids concentration was about25 g/L.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

The preparation of the SiO₂ sol was done by using tetraethylorthosilicate which was hydrolyzed in an ammonia based solution (0.5M)by stirring one hour at room temperature. The sol was then aged for 24hours, and larger particles or precipitates were removed by filtration.A hydroxide solution was then added with stirring. The concentrationSiO₂ was 30 g/L.

The preparation of the TiO₂/SiO₂ was done by mixing the sols in a weightratio of 70% TiO₂ and 30% SiO₂.

Dip coating was performed on the coupons (with the exception of theuntreated coupon) to introduce the nanoporous film onto the oxidizedlayer. Each coupon was dip coated into the desired sol and withdrawn ata speed of about 3 millimeters per second. Once removed from the dipcoating chamber, each coupon was dried at 100° C. for about 30 minutesand then cooled to room temperature in air. After the drying of thenanoporous coating was complete, each coated coupon was subjected to asintering step that included heating the coupon at 10° C. per minute to300° C. and then dwelled at 300° C. for 1 hour and then cooled to roomtemperature in air. This process resulted in a nanoporous film having athickness of between about 200 nanometers and about 300 nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for each of the five coupons described above. The tests weredone in aerated solutions at a temperature of 23° C. and a pH of 8.1.

The elements of the electrochemical cell included the following: asurrogate sea water solution (“Instant Ocean”) saturated with oxygen;temperature of 23° C.; V=300 mL; a Calomel Reference electrode; and a PtCounter Electrode=Pt. The working electrode was the coupon beingevaluated. The tested area of the working electrode was 18 cm². About 3centimeters of the working electrode was immersed into the sea watersolution. 300 mL of sea water solution was added to the cell, and theelectrodes inserted into the sea water solution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to thesea water solution, the OCV became quasi-constant for the coupons. Thepotential was then increased at a constant rate of 150 mV/s until thecurrent density reached I=3 mA/cm² or until the potential reached 0.9mV.

Referring now to FIG. 2, there is shown CV curves for each of thecoupons evaluated. FIG. 2 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The untreated coupon incurred a sharp increase in currentdensity of around 0.52 V. In contrast, the coupons having a nanoporouscoating as described herein incurred a sharp increase in current densityno less than about 0.68 V and as much as 0.8 V. Furthermore, the curveincrease associated with the coupons having a nanoporous coating is notas sharp as the curve for the uncoated coupon.

For the coupons having the nanoporous coating, the gradual increase inpotential is theoretically associated with nucleation of pits andformation of metastable pits. The sharp/spike increase in potential istheoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits. As such, theincrease in current density was associated with pitting and not with thetranspassive potential of the uncoated substrate material. Transpassivepotential is associated with the point in time when general dissolutionof the passivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the various metal oxides or mixturesof metal oxides provided significant protection from pitting as comparedto untreated coupons.

Example 3

Electrochemical testing and evaluation for pitting potential of carbonsteel coupons (B2 finish) immersed in sea water (Instant Ocean Solution)was conducted. The carbon steel coupons (B2 finish) had various surfacecoatings thereon as set forth below. Each coupon tested was a squareplate of either 1 inch by two inches or one inch by four inches.

The carbon steel (B2 finish) coupons evaluated were as follows: (1)untreated; (2) surface treated with a dry oxidation procedure plus acoating of Al₂O₃ (70%) and ZrO₂ (30%); and (3) surface treated with adry oxidation procedure plus a coating of Al₂O₃.

For the coupons treated with a dry oxidation step, the followingprocedure was used. The stainless steel coupon was heated to 200° C. ata rate of 10° C. per minute in air. The coupon was held at thistemperature for 1 hour and subsequently cooled to room temperature. Thisresulted in an oxidation layer having a thickness of about 100nanometers.

The preparation of the Al₂O₃ sol was done by hydrolysis of aluminumisopropoxide in a nitric acid solution including 1.43 mL of concentratednitric acid in 200 mL of water and 33 mL of Al(OPR)₄. A whiteprecipitate was obtained upon mixing the Al(OPr)₄ with the aqueous phasebecoming transparent due to peptization under strong stirring of thesuspension. The suspension became a transparent sol, and the sol wasplaced in a Spectra/Por dialysis tubing and dialyzed against pure water(MQ) to slowly adjust the pH to 3.5. The solids concentration was about25 g/L.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

Dip coating was performed on the coupons (with the exception of theuntreated coupon) to introduce the nanoporous film onto the oxidizedlayer. Each coupon was dip coated into the desired sol and withdrawn ata speed of about 3 millimeters per second. Once removed from the dipcoating chamber, each coupon was dried at 100° C. for about 30 minutesand then cooled to room temperature in air. After the drying of thenanoporous coating was complete, each coated coupon was subjected to asintering step that included heating the coupon at 10° C. per minute to300° C. and then dwelling (sometimes referred to as soaking) at 300° C.for 1 hour and then cooling to room temperature in air. This processresulted in a nanoporous film having a thickness of between about 200nanometers and about 300 nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for each of the five coupons described above. The tests weredone in aerated solutions at a temperature of 23° C. and a pH of 8.1.

The elements of the electrochemical cell included the following: asurrogate sea water solution (“Instant Ocean”) saturated with oxygen;temperature of 23° C.; V=300 mL; a Calomel Reference electrode; and a PtCounter Electrode=Pt. The working electrode was the coupon beingevaluated. The tested area of the working electrode was 18 cm². About 3centimeters of the working electrode was immersed into the sea watersolution. 300 mL of sea water solution was added to the cell, and theelectrodes inserted into the sea water solution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to thesea water solution, the OCV became quasi-constant for the coupons. Thepotential was then increased at a constant rate of 150 mV/s until thecurrent density reached I=3 mA/cm² or until the potential reached 0.9mV.

Referring now to FIG. 3, there is shown CV curves for each of thecoupons evaluated. FIG. 3 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The untreated coupon incurred a sharp increase in currentdensity of around −0.5 V. In contrast, the coupons having a nanoporouscoating as described herein incurred a sharp increase in current densityof about −0.35V.

For the coupons having the nanoporous coating, the gradual increase inpotential is theoretically associated with nucleation of pits andformation of metastable pits. The sharp/spike increase in potential istheoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits. As such, theincrease in current density was associated with pitting and not with thetranspassive potential of the uncoated substrate material. Transpassivepotential is associated with the point in time when general dissolutionof the passivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the various metal oxides or mixturesof metal oxides provided significant protection from pitting as comparedto untreated coupon.

Example 4

Electrochemical testing and evaluation for pitting potential ofstainless steel 201 (brushed) coupons immersed in deaerated 1M NaClsolution (pH=5.5) was conducted. The stainless steel 201 (brushed)coupons had various surface coatings thereon as set forth below. Eachcoupon tested was a square plate of either 1 inch by two inches or oneinch by four inches.

The stainless steel 201 (brushed) coupons evaluated were as follows: (1)surface treated with a wet oxidation procedure; and (2) surface treatedwith a wet oxidation procedure plus a coating of ZrO₂.

For the wet oxidation step, the following procedure was used. Thestainless steel 201 (brushed) coupon was immersed for 1 hour in a basic(pH=13) solution of hydrogen peroxide (6%) and then removed, washed withMQ water, and dried at room temperature in air. This resulted in anoxidation layer having a thickness of about 100 nanometers.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

Dip coating was performed on the coupon to introduce the nanoporous filmonto the oxidized layer. The coupon was dip coated into the ZrO₂ sol andwithdrawn at a speed of about 3 millimeters per second. Once removedfrom the dip coating chamber, the coupon was dried at 100° C. for about30 minutes and then cooled to room temperature in air. After the dryingof the nanoporous coating was complete, the coated coupon was subjectedto a sintering step that included heating the coupon at 10° C. perminute to 300° C. and then dwelled at 300° C. for 1 hour and then cooledto room temperature in air. This process resulted in a nanoporous filmhaving a thickness of between about 200 nanometers and about 300nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for both of the coupons described above. The tests were done indeaerated 1M NaCl solution at 23° C.

The elements of the electrochemical cell included the following: 1M NaCldeaerated solution; temperature of 23° C.; V=300 mL; a Calomel Referenceelectrode; and a Pt Counter Electrode=Pt. The working electrode was thecoupon being evaluated. The tested area of the working electrode was 18cm². About 3 centimeters of the working electrode was immersed into theNaCl solution. 300 mL of NaCl was added to the cell, and the electrodesinserted into the NaCl solution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to theNaCl solution, the OCV became quasi-constant for the coupons. Thepotential was then increased at a constant rate of 150 mV/s until thecurrent density reached I=3 mA/cm² or until the potential reached 0.9mV.

Referring now to FIG. 4, there is shown CV curves for each of thecoupons evaluated. FIG. 4 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The coupon having only the oxidation layer incurred a sharpincrease in current density of around 0.8 V. In contrast, the couponhaving an oxidation layer and a nanoporous coating as described hereinincurred a sharp increase in current density of about 0.31 V.

For the coupon having the nanoporous coating, the gradual increase inpotential is theoretically associated with nucleation of pits andformation of metastable pits. The sharp/spike increase in potential istheoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits. As such, theincrease in current density was associated with pitting and not with thetranspassive potential of the uncoated substrate material. Transpassivepotential is associated with the point in time when general dissolutionof the passivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the metal oxides provided significantprotection from pitting as compared to untreated (oxidation layer only)coupon.

Example 5

Electrochemical testing and evaluation for pitting potential ofstainless steel 436 (brushed) coupons immersed in deaerated 1M NaClsolution (pH=5.5) was conducted. The stainless steel 436 (brushed)coupons had various surface coatings thereon as set forth below. Eachcoupon tested was a square plate of either 1 inch by two inches or oneinch by four inches.

The stainless steel 436 (brushed) coupons evaluated were as follows: (1)surface treated with a wet oxidation procedure (“A”); (2) surfacetreated with a wet oxidation procedure (“B”); (3) surface treated with awet oxidation procedure plus a coating of ZrO₂(“C”); and (4) surfacetreated with a wet oxidation procedure plus a coating of ZrO₂ (“D”).

For the wet oxidation step, the following procedure was used. Thestainless steel 436 (brushed) coupon was immersed for 1 hour in a basic(pH=13) solution of hydrogen peroxide (6%) and then removed, washed withMQ water, and dried at room temperature in air. This resulted in anoxidation layer having a thickness of about 100 nanometers.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

Dip coating was performed on the coupon to introduce the nanoporous filmonto the oxidized layer. The coupon was dip coated into the ZrO₂ sol andwithdrawn at a speed of about 3 millimeters per second. Once removedfrom the dip coating chamber, the coupon was dried at 100° C. for about30 minutes and then cooled to room temperature in air. After the dryingof the nanoporous coating was complete, the coated coupon was subjectedto a sintering step that included heating the coupon at 10° C. perminute to 300° C. and then dwelled at 300° C. for 1 hour and then cooledto room temperature in air. This process resulted in a nanoporous filmhaving a thickness of between about 200 nanometers and about 300nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for all of the coupons described above. The tests were done indeaerated 1M NaCl solution at 23° C.

The elements of the electrochemical cell included the following: 1M NaCldeaerated solution; temperature of 23° C.; V=300 mL; a Calomel Referenceelectrode; and a Pt Counter Electrode=Pt. The working electrode was thecoupon being evaluated. The tested area of the working electrode was 18cm². About 3 centimeters of the working electrode was immersed into theNaCl solution. 300 mL of NaCl was added to the cell, and the electrodesinserted into the NaCl solution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to theNaCl solution, the OCV became quasi-constant for the coupons. Thepotential was then increased at a constant rate of 150 mV/s until thecurrent density reached I=3 mA/cm² or until the potential reached 0.9mV.

Referring now to FIG. 5, there is shown CV curves for each of thecoupons evaluated. FIG. 5 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The coupons having only the oxidation layer incurred a sharpincrease in current density of around 0.0 V. In contrast, the couponshaving an oxidation layer and a nanoporous coating as described hereinincurred a sharp increase in current density of about 0.15 V to about0.18 V.

For the coupons having the nanoporous coating, the gradual increase inpotential is theoretically associated with nucleation of pits andformation of metastable pits. The sharp/spike increase in potential istheoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits. As such, theincrease in current density was associated with pitting and not with thetranspassive potential of the uncoated substrate. Transpassive potentialis associated with the point in time when general dissolution of thepassivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the metal oxides provided significantprotection from pitting as compared to untreated (oxidation layer only)coupon.

Example 6

Electrochemical testing and evaluation for pitting potential of blackreinforcing steel rebar immersed in aerated 0.25M NaCl solutionsaturated with Ca(OH)2 was conducted. The black reinforcing steel rebarhad various surface coatings thereon as set forth below.

The black reinforcing steel rebar sample evaluated were as follows: (1)surface treated with 0.1 N HCl acid wash (“A”); (2) surface treated with0.1 N HCl acid wash (“B”); (3) surface treated with 0.1 N HCl acid washplus a coating of TiO₂ (“C”); and (4) surface treated with 0.1 N HClacid wash plus a coating of ZrO₂ (“D”).

The preparation of the TiO₂ sol was done by hydrolysis of titaniumisopropoxide in a nitric acid solution including 1.43 mL of concentratednitric acid in 200 mL of water and 33 mL of Ti(OPR)₄. A whiteprecipitate was obtained upon mixing the Ti(OPr)₄ with the aqueous phasebecoming transparent due to peptization under strong stirring of thesuspension. The suspension became a transparent sol, and the sol wasplaced in a Spectra/Por dialysis tubing and dialyzed against pure water(MQ) to slowly adjust the pH to 3.5. The solids concentration was about25 g/L.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

Dip coating was performed on the black rebar samples to introduce thenanoporous film onto the oxidized layer. The sample was dip coated intothe ZrO₂ sol and withdrawn at a speed of about 3 millimeters per second.Once removed from the dip coating chamber, the coupon was dried at 100°C. for about 30 minutes and then cooled to room temperature in air.After the drying of the nanoporous coating was complete, the coatedsample was subjected to a sintering step that included heating thesample at 10° C. per minute to 300° C. and then dwelled at 300° C. for 1hour and then cooled to room temperature in air. This process resultedin a nanoporous film having a thickness of between about 200 nanometersand about 300 nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for all of the samples described above. The tests were done inaerated 0.25M NaCl and saturated in Ca(OH)₂ solution and at 23° C.

The elements of the electrochemical cell included the following: 0.25 MNaCl deaerated solution saturated in Ca(OH)2; temperature of 23° C.;V=300 mL; a Calomel Reference electrode; and a Pt Counter Electrode=Pt.The working electrode was the sample being evaluated. The tested area ofthe working electrode was 18 cm². About 3 centimeters of the workingelectrode was immersed into the NaCl/Ca(OH)₂ solution. 300 mL ofNaCl/Ca(OH)₂ was added to the cell, and the electrodes inserted into thesolution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to theNaCl/Ca(OH)₂ solution, the OCV became quasi-constant for the samples.The potential was then increased at a constant rate of 150 mV/s untilthe current density reached I=3 mA/cm² or until the potential reached0.9 mV.

Referring now to FIG. 6, there is shown CV curves for each of thesamples evaluated. FIG. 6 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The samples that were subjected to the acid wash only incurred asharp increase in current density of around −0.1 V. In contrast, the onesample that was subjected to an acid wash and had a nanoporous coatingas described herein incurred a gradual increase in current densityaround 0.2 V and the second sample that was subjected to an acid washand had a nanoporous coating did not register an increase in current atall.

For the sample having the nanoporous coating of TiO₂, the gradualincrease in potential is theoretically associated with nucleation ofpits and formation of metastable pits. The sharp/spike increase inpotential is theoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits, with theexception of the sample coated with ZrO₂. As such, the increase incurrent density was associated with pitting and not with thetranspassive potential of the uncoated substrate material. Transpassivepotential is associated with the point in time when general dissolutionof the passivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the metal oxides provided significantprotection from pitting as compared to untreated (acid wash only)samples.

Example 7

Electrochemical testing and evaluation for pitting potential of aluminumcoupons immersed in sea water (Instant Ocean Solution) was conducted.The aluminum coupons had various surface coatings thereon as set forthbelow. Each coupon tested was a square plate of either 1 inch by twoinches or one inch by four inches.

The aluminum coupons evaluated were as follows: (1) surface treated witha wet oxidation procedure; and (2) surface treated with a wet oxidationprocedure plus a coating of a mixture of ZrO₂ and SiO₂.

For the wet oxidation step, the following procedure was used. Thealuminum coupon was immersed in a 0.1 N NaCl solution for 1 hour andthen removed, rinsed with MQ water and dried at room temperature. Thisresulted in an oxidation layer having a thickness of about 100nanometers.

The preparation of the ZrO₂ sol was done by the hydrolysis of Zrpropoxide in a nitric acid solution including 3 mL of concentratednitric acid in 150 mL of water and 11 mL of Zr(OPr)₄. After one day ofstrong stirring, a white suspension formed upon mixing the propoxide,and the aqueous phase became transparent due to peptization. Thetransparent sol was then dialyzed in MQ water until a pH of 3.2 wasobtained. The solids concentration was about 12 g/L.

The preparation of the SiO₂ sol was done by using tetraethylorthosilicate which was hydrolyzed in an ammonia based solution (0.5M)by stirring one hour at room temperature. The sol was then aged for 24hours, and the large particles or precipitates were removed byfiltration. A hydroxide solution was then added with stirring. Theconcentration SiO₂ was 30 g/L.

The preparation of the ZrO₂/SiO₂ was done by mixing the sols in a weightratio of 70% ZrO₂ and 30% SiO₂.

Dip coating was performed on the coupon to introduce the nanoporous filmonto the oxidized layer. The coupon was dip coated into the sol andwithdrawn at a speed of about 3 millimeters per second. Once removedfrom the dip coating chamber, the coupon was dried at 100° C. for about30 minutes and then cooled to room temperature in air. After the dryingof the nanoporous coating was complete, the coated coupon was subjectedto a sintering step that included heating the coupon at 10° C. perminute to 300° C. and then dwelled at 300° C. for 1 hour and then cooledto room temperature in air. This process resulted in a nanoporous filmhaving a thickness of between about 200 nanometers and about 300nanometers.

Anodic Polarization Measurements: Localized corrosion resistance wasmeasured for each of the two coupons described above. The tests weredone in aerated solutions at a temperature of 23° C. and a pH of 8.1.

The elements of the electrochemical cell included the following: asurrogate sea water solution (“Instant Ocean”) saturated with oxygen;temperature of 23° C.; V=300 mL; a Calomel Reference electrode; and a PtCounter Electrode=Pt. The working electrode was the coupon beingevaluated. The tested area of the working electrode was 18 cm². About 3centimeters of the working electrode was immersed into the sea watersolution. 300 mL of sea water solution was added to the cell, and theelectrodes inserted into the sea water solution.

Once the electrochemical cell setup was complete, the potentiostatstarted registering the open circuit potential (OCV) of the system.After approximately 1.5 hours of exposing the working electrode to thesea water solution, the OCV became quasi-constant for the coupons. Thepotential was then increased at a constant rate of 150 mV/s until thecurrent density reached I=3 mA/cm² or until the potential reached 0.9mV.

Referring now to FIG. 7, there is shown CV curves for each of thecoupons evaluated. FIG. 7 shows that the pitting potential (i.e., asharp increase in the current density) is dependent upon the presence orabsence of the nanoporous submicron metal oxide film as describedherein. The untreated coupon incurred a sharp increase in currentdensity of around −0.8 V. In contrast, the coupon having a nanoporouscoating as described herein incurred a sharp increase in current densityno less than about −0.55 V. The curve increase associated with thecoupon having a nanoporous coating is not as sharp as the curve for theuncoated coupon.

For the coupon having the nanoporous coating, the gradual increase inpotential is theoretically associated with nucleation of pits andformation of metastable pits. The sharp/spike increase in potential istheoretically associated with the growth of stable pits.

Visual inspection of the coated and uncoated coupons after the sharpincrease in current revealed the existence of some pits. As such, theincrease in current density was associated with pitting and not with thetranspassive potential of the uncoated substrate. Transpassive potentialis associated with the point in time when general dissolution of thepassivated layer occurs.

In conclusion, sintered nanoporous, sub-micron thin films derived fromdip-coated liquid sols comprising the mixture of metal oxides providedsignificant protection from pitting as compared to untreated coupons.

Example 8

In this Example, various heat exchanger plates (316 stainless steel)from two Plate Heat Exchangers (PHE) were evaluated for pittingcorrosion over an extended time after continuous exposure to sea water.The various heat exchanger plates were dip-coated three times in aliquid solution of titanium dioxide to form a titanium dioxidenanoporous film as described herein and compared to uncoated 316stainless steel coupons used as controls.

The first Plate Heat Exchanger (Model JR) contained 16 PHE plates(3″×19″). The first Plate Heat Exchanger was exposed to a seawatersolution having a pH of about 8.4 and a temperature of about 144° F.(62° C.) for a period of about 18 months. After 18 months, the firstPlate Heat Exchanger was removed from the solution, disassembled, andthe plates were inspected for wear (i.e., erosion and corrosion).

The second Plate Heat Exchanger (Model M3) contained 12 PHE plates(5″×16″). The second Plate Heat Exchanger was exposed to a seawatersolution having a pH of about 8.4 and a temperature of about 144° F.(62° C.) for a period of about one month. After one month, 12 moreplates (plates 13-24) were added to the second Plate Heat Exchanger andthe Exchanger was again exposed to the seawater solution for a period ofanother month. The second Plate Heat Exchanger was then briefly removedfrom the solution for a period of less than one week to determine if ashort lack of being in service would affect performance of the coating.It was then placed back in service and after an additional 3.5 months,the second Plate Heat Exchanger was disassembled and all 24 plates wereinspected for wear (i.e., erosion and corrosion). “Failure” is the termused herein to indicate that holes were detected in the plates.

The corrosion results for the plates of both the first Plate HeatExchanger and the second Plate Heat Exchanger, once removed from thesolution and disassembled, are shown in Table 1.

TABLE 1 First Plate Heat Second Plate Heat Exchanger (18 months)Exchanger (4-5 months) Plate Number Visual Inspection Plate NumberVisual Inspection 1 No corrosion 1 No corrosion 2 No corrosion 2 Nocorrosion 3 No corrosion 3 No corrosion 4 Slight wear 4 No corrosion 5No corrosion 5 No corrosion 6 No corrosion 6 No corrosion 7 No corrosion7 No corrosion 8 Slight wear 8 No corrosion 9 No corrosion 9 Nocorrosion 10 No corrosion 10 No corrosion 11 Slight wear 11 No corrosion12 No corrosion 12 No corrosion 13 No corrosion 13 No corrosion 14 Nocorrosion 14 No corrosion 15 No corrosion 15 No corrosion 16 Nocorrosion 16 No corrosion 17 Failure 18 No corrosion 19 No corrosion 20No corrosion 21 No corrosion 22 No corrosion 23 No corrosion 24 Nocorrosion

As shown in Table 1, 13 of the 16 plates in the first Plate HeatExchanger showed no signs of corrosion. Moreover, while one plate in thesecond Plate Heat Exchanger failed, all other plates showed no signs ofcorrosion. It should be noted that the one plate, plate number 17, wasactually not coated with the liquid solution of titanium dioxide.

The first and second Plate Heat Exchangers were then reassembled andagain exposed to a seawater solution having a pH of about 8.4 and atemperature that ranged from about 120° F. (49° C.) to about 140° F.(60° C.). The first and second Plate Heat Exchangers were exposed forthe periods as indicated in Table 2, and then again disassembled. Theplates were subjected to dye penetrant testing to detect any cracks andpin holes and inspected for wear. The results are shown in Table 2.

TABLE 2 First Plate Heat Exchanger (26 months) Second Plate Heat DyePenetration Exchanger (13 months) Plate Number Test Plate Number VisualInspection 1 No holes 1 No holes 2 No holes 2 No holes 3 No holes 3 Noholes 4 No holes 4 No holes 5 No holes 5 No holes 6 No holes 6 No holes7 Failed 7 No holes 8 No holes 8 No holes 9 No holes 9 No holes 10 Noholes 10 No holes 11 No holes 11 No holes 12 No holes 12 No holes 13 Noholes 13 No holes 14 No holes 14 No holes 15 No holes 15 No holes 16 Noholes 16 No holes 17 Removed 18 No holes 19 No holes 20 No holes 21 Noholes 22 No holes 23 No holes 24 No holes

As shown in Table 2, 15 of the 16 plates in the first Plate HeatExchanger showed no signs of holes (pitting). Moreover, none of the 23plates left in the second Plate Heat Exchanger showed the presence ofholes.

After inspection, the titanium dioxide-coated plates were cut into 1 cm²pieces, washed, and sputtered (250 mA) with platinum for a period ofabout one minute. The pieces were then examined using a LEO SEM under500,000 magnification. It was found that most of the titanium dioxidenanoparticles had a diameter in the range of from about 6 nm to about 12nm, and the inter particle pores had a diameter of about 4 nm.

We Claim:
 1. A method of making a metal-containing apparatus, the methodcomprising: providing a substrate material constructed of a metal;introducing an oxidized layer on a surface of the substrate material,the oxidized layer having a thickness of less than about 200 nanometers;providing a sol comprising nanoparticulate γ-AlOOH, TiO₂, SiO₂, ZrO₂, ora mixture thereof, the nanoparticulates having a primary particle sizeof from about 1.5 nanometers to about 50 nanometers; contacting the solwith the oxidized layer to provide at least one layer of the sol ontothe oxidized layer; heating the at least one layer at a temperature offrom about 60° C. to about 100° C. for a time period of from about 15minutes to about 2 hours to produce at least one dry layer; andthermally sintering the at least about one dry layer to produce ananoporous film comprising oxygen and aluminum, titanium, silicon,zirconium or a combination thereof, the nanoporous film having athickness of less than about 1 micrometer and a porosity of from about26% to about 80%.
 2. The method of claim 1, wherein the oxidized layeris introduced onto the substrate using a heating process, a basicsolution, or a hydrogen peroxide solution.
 3. The method of claim 1,wherein the sol is contacted with the oxidized layer utilizing a processselected from the group consisting of dipping, draining, spraying,electro-deposition, spinning, and combinations thereof.
 4. The method ofclaim 1, wherein the substrate material is constructed of a metalselected from the group consisting of stainless steel, carbon steel, andaluminum.
 5. The method of claim 1, wherein the nanoporous film has athickness of from about 0.01 micrometers to about 1 micrometer.
 6. Themethod of claim 1, wherein the nanoporous film has a porosity of about26%.
 7. The method of claim 1, wherein the sol comprises nanoparticulateTiO₂ having a primary particle size of from about 3 nanometers to about8 nanometers.
 8. The method of claim 1, wherein the sol comprisesnanoparticulate SiO₂ having a primary particle size of from about 1.4nanometers to about 8 nanometers.
 9. The method of claim 1, wherein thesol comprises a mixture of nanoparticulate SiO₂ having a primaryparticle size of from about 1.5 nanometers to about 8 nanometers andnanoparticulate TiO₂ having a primary particle size of from about 3nanometers to about 8 nanometers.
 10. The method of claim 1, wherein thesol comprises nanoparticulate ZrO₂ having a primary particle size offrom about 3 nanometers to about 8 nanometers.
 11. The method of claim1, further including a second nanoporous film on top of the firstnanoporous film, the second nanoporous film comprising oxygen and anelement selected from the group consisting of aluminum, titanium,silicon, zirconium and combinations thereof, the second nanoporous filmhaving a thickness of less than about 1 micrometer and having a porosityof from about 26% to about 80%.
 12. The method of claim 11, wherein thesecond nanoporous film comprises zirconium dioxide.
 13. The method ofclaim 11, wherein the second nanoporous film has a thickness of fromabout 0.01 micrometers to about 1.0 micrometers.